In recent years, there has been the spread of devices that applies compression encoding to an image by employing an encoding format for handling image information as digital signals, taking advantage of redundancy peculiar to the image information, aiming for transmission and storage of high effective information at that time, and compressing the image by orthogonal transform such as discrete cosine transform or the like and motion compensation. Examples of this encoding format include Moving Picture Experts Group (MPEG) and the like.
Especially, MPEG2 (ISO/IEC 13818-2) is defined as a general-purpose image encoding format, and is a standard encompassing both of interlaced scanning images and sequential-scanning images, and standard resolution images and high definition images. For example, MPEG2 has been widely used in a broad range of applications for professional usage and for consumer usage. By employing the MPEG2 compression format, a code amount (bit rate) of 4 to 8 Mbps is allocated in a case of an interlaced scanning image of standard resolution having 720×480 pixels, for example. By employing the MPEG2 compression format, a code amount (bit rate) of 18 to 22 Mbps is allocated in a case of an interlaced scanning image of high resolution having 1920×1088 pixels, for example. Thus, high compression rate and excellent image quality can be realized.
MPEG2 has been principally used for high image quality encoding adapted to broadcasting usage, but have not handled an encoding format of a lower code amount (bit rate) than the code amount of MPEG1, i.e., an encoding format having a higher compression rate. With the widespread of mobile terminals, growing needs for such an encoding format is expected from now on, and the MPEG4 encoding format has been standardized in response to the needs. With regard to an image encoding format, the specification thereof was confirmed as an international standard as ISO/IEC 14496-2 in December, 1998.
As for the schedule of standardization, H.264 and MPEG-4 Part 10 (Advanced Video Coding, hereafter referred to as H.264/AVC) became an international standard in March, 2003.
Further, as an expansion of H.264/AVC, standardization of FRExt (Fidelity Range Extension), which includes encoding tools necessary for operations such as RGB, 4:2:2, and 4:4:4, and MPEG-2 stipulated 8×8 DCT and quantization matrices, has been completed in February, 2005. Accordingly, the encoding format can favorably express film noise included in movies using H.264/AVC, and is to be used in a wide range of applications such as Blu-Ray Disc (registered trademark).
However, recently, there are increased needs for further higher compression encoding, such as to compress images around 4000×2000 pixels, which is four times Hi-Vision images, or to distribute Hi-Vision images in an environment with limited transmission capacity like the Internet. Therefore, the VCEG (=Video Coding Expert Group) under the ITU-T, described above, is continuously studying improvement of encoding efficiency (see Non-Patent Document 1).
Currently, for the purpose of further improvement of the encoding efficiency from H.264/AVC, Joint Collaboration Team-Video Coding (JCTVC), which is a joint standardization body of the ITU-T and the ISO/IEC, has moved forward with standardization of an encoding format called high efficiency video coding (HEVC). In HEVC, Non-Patent Document 2 has been issued as a draft.
By the way, in H.264/AVC, when reference image indexes are allocated in order to decode a B-picture, reference image indexes are allocated to a list L0 in descending order of picture order count (POC) that is information indicating output order of pictures. The reference image indexes are allocated to a list L1 in ascending order of POC.
Further, when the reference image indexes are allocated in order to decode a P-picture, the reference image indexes are allocated in decoding order.